1. Field of the Invention
This invention relates to the desulfurization of molten pig iron employing a desulfurization agent which comprises calcium carbide and a solid material which decomposes or evaporates at the temperatures of the molten pig iron to be desulfurized to produce a gas which desulfurization agent is introduced into the molten pig iron at a relatively high rate of between 70 and 250 kilograms per minute and at a relatively low gas/solid ratio. More particularly, this invention relates to the desulfurization of molten pig iron employing a fluidized stream of a desulfurizing agent which is a mixture of calcium carbide and a decomposable or evaporatable solid material which produces a gas at the temperature of molten pig iron.
2. Discussion of the Prior Art
For the last two decades it has become common to carry out external desulfurization of hot metal. Because of rising prices of coke it has become necessary in the last ten years to intensify efforts to reduce coke rates. This has led to the development of new technologies for injecting fuels in the blast furnace and at the same time decreasng the basicity of the slag. As a result the sulfur content of the hot melt has increased making it necessary to intensify external desulfurization techniques. A further reason for carrying out such external desulfurization lies in the rising demands for hot metal with low sulfur content for the steel shop in order to meet the requirements of the consumer.
The injection of desulfurizing agents such as calcium carbide into a molten melt bath to effect desulfurization is broadly known. In parent Application Ser. No. 270,397 there is described an apparatus which permits the fluidization of such a desulfurization agent and the injection of the same into the metal bath through a lance which passes through the surface of the molten metal.
It has become desirable to provide a means by which the sulfur content is reduced to as low as possible and by which the resultant steel has a consistently low sulfur content. Heretofore it was believed that the molten pig iron could be desulfurized employing a fluidized stream of solid which was injected into the molten metal at a rate exceeding 25 feet per second at the point of delivery while utilizing a stream having an apparent density of at least 1 pound per cubic foot (U.S. Pat. No. 3,001,864 to Muller et al.). Unfortunately, such a technique has not been accepted by the industry inasmuch as the molten pig iron has not been adequately desulfurized.
Other procedures for the desulfurization of molten metals have utilized inert gases to effect fluidization of the desulfurization agent, these gases being materials such as nitrogen, argon, helium and the like. The rate at which the calcium carbide has been fed to the molten metal has been around 6.5 lbs. of calcium carbide per minute (U.S. Pat. No. 2,803,533). It is also been proposed to conduct the desulfurization of hot metal using a variety of process conditions including an injection rate of desulfurization agent into the molten pig iron of about 120 lbs. per minute.
Thusfar, none of these techniques have provided a commercially feasible method of desulfurizing molten pig iron. The reason lies in the fact that when injecting calcium carbide in melts, it is necessary that the liquid metal be mixed through the melt sufficiently in order to reach a maximum sulfur decomposition, since the calcium carbide particles swim up to the surface very quickly in the melt due to the great difference in density and are thus withdrawn from the reaction.
To increase this mixing, a desulfurizing agent has already been proposed, which consists of calcium carbide and a gas-separating addition (U.S. Pat. No. 3,598,573). However, it has proven that the use of this desulfurization agent alone is not sufficient in and of itself for maximum desulfurization improvement.
One object of the invention, therefore, is to provide a desulfurization process with which melts can be treated with a high utilization degree of the desulfurization agent, and which, moreover, permits the exact adjustment of any sulfur content in the melt even to extremely low levels. Another object of the invention is to provide a process which permits the injection of large amounts of desulfurization agent in a relatively short time without impairing the desulfurization effectiveness, in order to be thus able to desulfurize large amounts of pig iron without disturbing the operation.